The electrode layer in proton exchange fuel cells (PEFC's) is typically coated from an ink that includes a Pt or Pt-alloy catalyst dispersed on a carbon black support, a perfluorosulfonic acid ionomer (PFSA) and alcohol-water solvent. The carbon black support provides gas transport for reactants to the catalyst and product water to the flow channel, while the PFSA ionomer provides proton conduction to the catalyst as well as binding of the porous carbon network1.
Through-layer cracks, however, can develop during solvent drying of the coated ink film which directly impacts durability of the fabricated membrane-electrode assembly (MEA) during fuel cell operation2. As an example, FIG. 1 shows the polymer flow and resulting thickness reduction that occurs in a 25 μm thick NAFION® membrane at an electrode crack after humidity cycling. Cell failure occurs once a through-layer crack is formed in the separator membrane due to reactant gas leakage between anode and cathode layers.
The electrode layer has a high porosity at ˜70% v/v for optimal gas transport that thereby carries a weak fracture resistance. As a result, the coated ink film is susceptible to through-layer crack formation during solvent drying. In addition to a uniform tensile stress derived from the solvent capillary pressure within the consolidated carbon black mesopore volume, a local tensile stress can also develop from uneven permeation of the ink ionomer solution into a porous coating substrate3-9. Both stresses are typically present when the electrode ink is coated directly on gas-diffusion-media (CCDM or catalyst-coating-on-diffusion-media) which then requires a mechanical reinforcement of the fragile carbon microstructure to avoid crack formation during solvent-drying.
Accordingly, there is a need for methods of improving fuel cell membranes by reducing the electrode mudcracking.